In a high intensity discharge (HID) lamp, a medium to high pressure ionizable gas, such as mercury or sodium vapor, emits visible radiation upon excitation typically caused by passage of current through the gas. One class of HID lamps comprises electrodeless lamps which generate an arc discharge by generating a solenoidal electric field in a high-pressure gaseous lamp fill. In particular, the lamp fill, or discharge plasma, is excited by radio frequency (RF) current in an excitation coil surrounding an arc tube. The arc tube and excitation coil assembly acts essentially as a transformer which couples RF energy to the plasma. That is, the excitation coil acts as a primary coil, and the plasma functions as a single-turn secondary. RF current in the excitation coil produces a time-varying magnetic field, in turn creating an electric field in the plasma which closes completely upon itself, i.e., a solenoidal electric field. Current flows as a result of this electric field, resulting in a toroidal arc discharge in the arc tube.
At room temperature, the solenoidal electric field produced by the excitation coil is typically not high enough to ionize the gaseous fill and thus initiate the arc discharge. One way to overcome this shortcoming is to lower the gas pressure of the fill, for example, by first immersing the arc tube in liquid nitrogen so that the gas temperature is decreased to a very low value and then allowing the gas temperature to increase. As the temperature rises, an optimum gas density is eventually reached for ionization, or breakdown, of the fill to occur so that an arc discharge is initiated. However, the liquid nitrogen method of initiating an arc discharge is not practical for widespread commercial use.
More recent methods for starting electrodeless HID lamps entail using starting aids to capacitively couple the high voltage developed across the excitation coil turns into the arc tube. As a result of this voltage gradient, a capacitive current flows between the starting aid and the excitation coil, and hence through the arc tube, thereby ionizing the gaseous fill and producing a low current discharge therein. When the gas is sufficiently ionized, a transition is made from a relatively low current discharge to a relatively high current, high intensity solenoidal arc discharge. Such a starting aid may comprise, for example, a pair of capacitive starting electrodes, as described in U.S. Pat. No. 4,902,937 of H.L. Witting, issued Feb. 20, 1990, and assigned to the instant assignee, which patent is incorporated by reference herein. Each starting electrode comprises a conductive ring which surrounds the arc tube and is connected to the excitation coil of the HID lamp. Coupling a high voltage signal between the pair of starting electrodes causes an electric field to be produced therebetween which is of sufficient magnitude to create a discharge in the arc tube due to the arc tube wall capacitance. Furthermore, as it has been determined that the application of relatively large electric fields directly to the arc tube via the starting aid may cause early arc tube degradation, heat sensitive members, e.g. bimetallic strips, are utilized for moving the starting electrodes away from the arc tube after initiating an arc discharge, thereby preserving the useful life of the lamp.
A spiral starting electrode for an electrodeless HID lamp is described in U.S. Pat. No. 4,894,590 of H.L. Witting, issued Jan. 16, 1990, and assigned to the instant assignee, which patent is incorporated by reference herein. A single, conical-spiral-shaped starting electrode is positioned so that its narrower end is adjacent to, or on, the arc tube surface. The wider end of the starting electrode is positioned so that flux generated by the excitation coil cuts the turns of the spiral electrode, thereby generating a high voltage signal which results in a sufficiently high electric field gradient to create a discharge in the arc tube. A bimetallic strip is utilized to move the starting electrode away from the arc tube after an arc discharge is initiated therein.
Starting electrodes which are moved from a rest position to a starting location adjacent to the arc tube by piezoelectric means are described in U.S. Pat. No. 4,894,589 of J.C. Borowiec, issued Jan. 16, 1990, and assigned to the instant assignee, which patent is incorporated by reference herein. The piezoelectric means is deactivated after an arc discharge is initiated, and the starting electrodes are moved back to the rest position. The piezoelectric means allows selective movement of the starting electrodes, thereby enabling the lamp to be restarted, if necessary, even if the arc tube is still hot.
In commonly assigned, copending U.S. patent application of H.L. Witting, Ser. No. 417,404, filed Oct. 5, 1989, another starting aid for an electrodeless HID lamp is described which comprises a first conductive coil disposed about a second conductive coil, each coil having a truncated-cone shape. The coils are wound in opposite directions so that voltages induced therein add together to provide a sufficiently high electric field gradient to initiate an arc discharge in the arc tube. A bimetallic support is used to move the starting aid between a starting position adjacent to the arc tube and a lamp-operating position at a predetermined location away from the arc tube. The Witting application is incorporated by reference herein.
Although each of the hereinabove described movable starting aids is effective in initiating an arc discharge in an electrodeless HID lamp, it may be desirable in some applications to simplify lamp construction by employing a fixed starting aid. Such a fixed starting aid may comprise, for example, electrodes extending into the arc tube, such as those described in commonly assigned U.S. Pat. No. 4,959,584 of J.M. Anderson, issued Sept. 25, 1990, which patent is incorporated by reference herein. Although such starting electrodes are effective in starting an HID lamp, a disadvantage of such fixed electrodes is that even after the starting voltage is removed therefrom, the electrodes act as small antennae, picking up RF electric fields which cause small currents to flow between the arc discharge and the starting aid. These currents have a detrimental effect on the arc tube. Therefore, to be practical, such a fixed starting aid must include means for substantially reducing, or eliminating, currents flowing between the starting aid and the arc tube during normal lamp operation, thereby extending the useful life of the lamp.